Our project's purpose was to design and construct a bridge that will support the greatest weight possible. The specific rules to this is that each team was given only toothpicks, skewers and straws as supplies as well as glue. The bridge must be free standing, have a length of 40-50 cm and a width of 3-10 cm. The most amount of layers of materials stacked together allowed was four. On the day of testing, a bar will be placed across the bridge, with a bucket hanging from the bar by a rope. Each team will add weight to their own bucket in whatever increment they choose until the bridge collapses. The bridge must support the most recent weight added for ten seconds before it can be counted.
In order to score each project, the mass of the weight supported was divided by the mass of the bridge, times 100. This means if a light bridge and a heavy bridge support the same weight, the lighter bridge wins. Additionally, the weight cannot fall below 90% of its original height.
It is important to realize what forces are acting upon our bridge to build one that will hold large amounts of weight. Such forces include tension, compression, gravity, torque, torsion, and more. A sturdy bridge will be able to withstand strong forces relative to its weight.
So what do all these pictures mean? Well...
The first picture explains tension and compression. The arrows pointing towards each other represent compression and the arrows away from each other represent tension, or stretching.
The second picture is another view of these forces, as well as shearing. Shearing is when two unequal forces push in opposite directions.
The third picture here demonstrates torsion. Torsion is similar to torque, it twists the bridge and can be detrimental.
The fourth picture shows torque. Torque is the tendency of a force to rotate an object, so rotation. A bridge can experience this and can even cause the bridge to nearly flip, resulting in a collapse.
The last picture is an example of Hooke's Law. The spring is being stretched, and, like a bridge, can only withstand so much stretching until it deforms. If you keep stretching the object passed its point of deformation (where it will no longer revert to normal if the force is stopped), the object will break.
With this information, my group started testing what materials would be able to hold the most weight. We found straws to be extremely weak almost immediately. They held about 16N vertically and 3N horizontally. Toothpicks weighed about 1g each, and held 55N vertically and 21N horizontally. These were good for making our bridge fit within the budget. Each skewer weighed 14g and held the same amount as the toothpicks on record, but we could only measure up to 55N so both the toothpick and the skewer exceeded this. We glued three skewers together and hung weights from the unified rod. This held a large amount of weight, and we decided to base our bridge off of a design incorporating three skewers together.
After some calculations for our budget (we were given a budget of $2500, where each skewer was $97, each toothpick was $18, and each straw was $55) We decided budget-wise the best idea would be to use twelve skewers and make them into four bundles, and for the rest of our bridge use toothpicks. This meant the base of our bridge would be very strong, and the truss would be made of toothpicks. We did not, however, take into account the weight of our bridge. This would end up being our Achilles heel, but more of that in a bit.
Our bridge had a total cost of $2028, with $840 in toothpicks and $1164 in skewers (this amounts to 66 toothpicks and 12 skewers) We did not use straws.
The actual making of our bridge was simple enough. We cut the ends off of skewers and glued three together so as to make one thick skewer. With these on the base of the bridge going across, we put triangles of toothpicks on top going across on either side. We also added a truss under the bridge, but we used less toothpicks than the top due to budget constraints. There were four triangles on each side of the bridge of the truss on top, and one on the bottom. My group was not entirely sure what would happen with the truss, but it looked like a solid design so we went with it.
Our bridge before being tortured
Our bridge after. You can see the top is bent but the rest of the bridge looks to be in decent condition.
Our competitor's bridge was more flexible, which under the conditions of the competition, worked well for them. They held less weight and started bending quicker, but due to the lightness of their bridge and our score system their bridge won. In reality a bridge that bent that quick would not be very good, but our rules were different. On a second try, my team would probably try to make a lighter bridge that better resisted compression, but more on that in a moment.
Our competitor's bridge, in all its unholiness. It bent easily but did not snap quickly. Their's held up 2620 grams.
We think that the other team's bridge better withstood the sideways forces acting upon than our own bridge. While their's may have started to bend immediately, it was able to take the downward force with its flexibility. Our bridge could take the downward force better, but this left little room for anything sideways, which tore the top truss apart.
If we had another chance to build our bridge, there's a few things my team would change. First and foremost, we would make the bridge better able to withstand sideways forces. We would also place the weight in a different area, as the rope of the bucket was placed at an awkward angle. This put extra force on the top truss, which ended up breaking. We also would probably make our bridge lighter, as we would have an advantage with the way the scoring system works. I think our design was decent, it just needs a bit of tweaking.
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